1. Technical Field
The present invention provides a catalyst system for the polymerization of propylene to produce highly crystalline polypropylene that includes the combination of a conventional supported Ziegler-Natta catalyst with an electron donor compound having the general formula SiRm(ORxe2x80x2)4-m where R is selected from the group consisting of an alkyl group, a cycloalkyl group, an aryl group and a vinyl group; Rxe2x80x2 is an alkyl group; and m is 0-3, wherein when R is an alkyl group, R may be identical with Rxe2x80x2; when m is 0, 1 or 2, the Rxe2x80x2 groups may be identical or different; and when m is 1, 2 or 3, the R groups may be identical or different. The electron donor preferably has the formula: 
wherein R1 is an alkyl or cycloalkyl group containing a primary, secondary or tertiary carbon atom attached to the silicon atom; R2 and R3 alkyl or aryl groups; and R4 is an alkyl or cycloalkyl group with a secondary or tertiary carbon atom attached to the silicon atom, R1 and R4 can be the same or different and R2 and R3 can be the same or different.
2. Description of Prior Art
Catalyst systems for the polymerization of olefins are well known in the art. Typically, these systems include a Ziegler-Natta type polymerization catalyst; a co-catalyst, usually an organoaluminum compound; and an electron donor, usually an organosilicon compound. Examples of such catalyst systems are shown in the following U.S. Pat. Nos. 4,107,413; 4,2394,721; 4,439,540; 4,115,319; 4,220,554; 4,460,701; and 4,562,173; the disclosures of these patents are hereby incorporated by reference. These are just a few of the scores of issued patents relating to catalysts and catalyst systems designed primarily for the polymerization of propylene and ethylene.
A conventional supported Ziegler-Natta polymerization catalyst generally comprises a complex derived from a halide of a transition metal, for example, titanium, chromium or vanadium with a metal hydride and/or a metal alkyl that is typically an organoaluminum compound. The catalyst is usually comprised of a titanium halide supported on a magnesium compound complexed with an alkyaluminum. An external electron donor or selectivity control agent (SCA) can be added to improve the stereoregulation of the catalyst so that a more crystalline polymer is produced.
The development of these polymerization catalysts has improved catalyst and polymer properties. In addition to the improved catalysts, improved activation methods have also lead to increases in the catalyst efficiency. One improved activation method includes a process for pre-polymerizing the catalyst just prior to introducing the catalyst into the reaction zone. One such pre-polymerization process is disclosed in U.S. Pat. No. 4,767,735, the disclosure of which is hereby incorporated by reference.
In addition to the development of new catalysts and new reaction processes, a discovery of an appropriate electron donor to go with a supported Ziegler-Natta catalysts in forming a total catalyst system has been found to be of great benefit to the polymerization art in leading to dramatic improvements in the catalyst and polymer properties. In such a catalyst system, it is believed that a co-catalyst activates the catalyst and provides the initiation of a polymer chain. A co-catalyst that works well with a supported Ziegler-Natta catalyst component is an organoaluminum compound, usually an alkyaluminum and most typically a trialkylaluminum, such as triethylaluminum (TEA) or triisobutylaluminum (TIBAI). Examples of other organoaluminum compounds useful as a co-catalyst for a conventional supported Ziegler-Natta catalyst component include an alkylaluminum dihalide and a dialkylaluminum halide.
An electron donor compound is used in the polymerization reaction to reduce the atactic form of the polymer thereby giving control of and increasing the production of stereoregular or crystalline polymers. Although a broad range of compounds are known generally as electron donors, a particular catalyst may have a specific compound or group of compounds with which it is especially compatible. Discovery of an appropriate type of electron donor which gives good catalyst efficiency as well as improved control of the isotactic index of the desired polymer product and other properties of the product would be highly advantageous. One such group of electron donors are disclosed in U.S. Pat. No. 4,927,797.
The present invention comprises another such discovery. It has been surprisingly discovered that a specific group of silane compounds serving as electron donors in combination with a particular type of catalyst results in significant increases in crystallinity of the polymer produced over the previously known crystallinity for this particular type of catalyst as well as other known catalyst systems.
The present invention provides a catalyst system for the polymerization of propylene wherein the system includes the combination of a particular type of catalyst with a specific group of electron donor compounds which results in significant increases in the crystallinity of the polymer product. The catalyst comprises a conventional supported Ziegler-Natta catalyst component, an organoaluminum co-catalyst and an organosilicon electron donor. This combination results in a catalyst system that produces a polymer product with significantly higher crystallinity (lower xylene solubles) than provided by previous catalyst systems as illustrated by the comparative examples included below. Further, the catalyst system of the present invention using a particular conventional supported Ziegler-Natta catalyst component provides better control of the xylene solubles of the polymer product than with other types of catalyst components. Further, the catalyst system of the present invention using a particular electron donor provides better control of the xylene solubles of the polymer product than with other types of electron donors. These and other beneficial advantages will become more apparent from the following detailed description of the invention and the accompanying examples.
The above-described catalyst exhibits some increase in crystallinity of the polymer product over previously known catalysts when it is paired with a previously used electron donor, but that increase is significantly greater when the catalyst is paired with electron donors as described by the present invention. The invention provides improved operational control of the properties of the polymer product such as the xylene solubles.
The most significant unexpected result obtained from the combination of the catalyst described in U.S. Pat. Nos. 4,784,983 and 4,861,847 and CMDS, DIDS, DTDS, CPDS and CIDS is the dramatic increase in the crystallinity of the polymer produced by the catalyst system in the polymerization of propylene. The catalyst system has been shown to achieve efficiencies higher than 30 kg/g-catxc2x7h for Si/Ti mole ratios within the range 4-200.
The invention also provides a process for the polymerization of propylene. The process comprises:
forming a catalyst by contacting a supported Ziegler-Natta catalyst component with an organoaluminum compound, preferably with a trialkylaluminum;
contacting the catalyst with an electron donor either simultaneously with or after contact with the organoaluminum, the electron donor being an organosilicon compound as described by the formula above,
optionally, pre-polymerizing the catalyst by contacting a small amount of monomer with the catalyst;
introducing the catalyst into a reaction zone under polymerization conditions containing the monomer and, optionally additional amounts of organoaluminum compound and the electron donor; and
withdrawing a polymer product.
If the monomer is propylene, the xylene solubles are within the range of 0.6-3.00 wt %, while the Si/Ti ratio in the reaction zone is within the range 4-200. If the electron donor is dicyclopentyldimethoxysilane and the monomer is propylene, the crystallinity as defined by the isotacticity (per cent pentad distribution of the xylene insoluble fraction) of the polymer product is equal to or greater than 98%.
The present invention relates to the combination of a particular group of compounds as electron donors with a particular type of catalyst for use in the polymerization of propylene.
It has been discovered that a particular group of electron donors does significantly enhance the catalytic properties of a conventional supported Ziegler-Natta catalyst. Any of the conventional Ziegler-Natta transition metal compound catalyst components can be used in the present invention. Electron donors are typically used in two ways in the formation of a Ziegler-Natta catalyst and a catalyst system. First, an internal electron donor may be used in the formation reaction of the catalyst as the transition metal halide is reacted with the metal hydride or metal alkyl.
The internal electron-donor compounds suitable for preparing conventional Ziegler-Natta catalyst components include ethers, eketones, lactones, electron donors compounds with N, P and/or S atoms and specific classes of esters. Particularly suitable are the esters of phthalic acid, such as diisobutyl, dioctyl, diphenyl and benzylbutylphthalate; esters of malonic acid, such as diisobutyl and diethylmalonate; alkyl and arylpivalates; alkyl, cycloalkyl and arylmaleates; alkyl and aryl carbonates such as diisobutyl, ethyl-phenyl and diphenylcarbonate; succinic acid esters, such as mono and diethyl succinate. These esters of phthalic acid are the preferred donors.
The second use for an electron donor in a catalyst system is as an external electron donor and stereoregulator in the polymerization reaction. The same compound may be used in both instances, although typically they are different. A common external electron donor is an organic silicon compound, for example, cyclohexylmethyl dimethoxysilane (CMDS). A description of the two types of electron donors is provided in U.S. Pat. No. 4,535,068, the disclosure of which is hereby incorporated by reference.
The conventional Ziegler-Natta catalyst component comprises a compound of the general formula MR+x where M is the metal, R is a halogen or a hydrocarboxyl and x is the valence of the metal. Preferably, M is a transition metal, more preferably, Group IVB, VB or VIB metal, most preferably a Group IVB, and specifically titanium. Preferably, R is chlorine, bromine, an alkoxy or a phenoxy, more preferably chlorine or ethoxy and most preferably, chlorine. Illustrative examples of the transition metal compound catalyst components are TiCl4, TiBr4, Ti(OC2H5)3Cl, Ti(OC2H5)3Cl, Ti(OC3H7)2Cl2, TiOC6H13)2Cl2, Ti(OC2H5)2Br2 and Ti(OC12H25)Cl3. Mixtures of the transition metal compounds may be used. No restriction on the number of transition metal compounds is made as long as at least one transition metal compound is present.
The support should be an inert solid which is chemically unreactive with any of the components of the conventional Ziegler-Natta Catalyst. The support is preferably a magnesium compound. Examples of the magnesium compounds which are to be used to provide a support source for the catalyst component are magnesium halides, dialkoxymagnesiums, alkoxymagnesium halides, magnesium oxyhalides, dialkylmagnesiums, magnesium oxide, magnesium hydroxide, and carboxylates of magnesium.
The organoaluminum compound which acts as a co-catalyst is preferably an alkylaluminum of the general formula AlRxe2x80x23 where Rxe2x80x2 is an alkyl of from 1-8 carbon atoms or a halogen, Rxe2x80x2 being the same or different with at least one R1 being an alkyl and Rxe2x80x2 being the same or different. Preferably, the organoaluminum compound is an alkylaluminum, an alkylaluminum dihalide or a dialkylaluminum halide. More preferably, Rxe2x80x2 is an alkyl making the alkyaluminum a trialkylaluminum. Most preferably, the alkylaluminum is an trialkylaluminum such as triethylaluminum (TEAI) or triisobutylaluminum (TIBAI).
One particular Ziegler-Natta catalyst component which can be used in the present invention is a commercially available Ziegler-Natta catalyst component for the polymerization of olefins comprising:
a catalyst component consisting essentially of about 1.5-6.0% by weight or titanium, about 10-20% by weight of magnesium, about 40-70% by weight of a halide selected from the group consisting of chlorine, bromine, and iodine; about 5-25% by weight of a polycarboxylic ester selected from the group consisting of the esters of aliphatic, aromatic and alicyclic polycarboxylic acids; and about 0.1-2.5% by weight of an organic phosphorus compound selected from the group consisting of hydrocarbon esters of phosphoric acid and phosphorus acid wherein each hydrocarbon group has 1 to 6 carbon atoms, said catalyst component being prepared by a process having the following steps:
a) dissolving in a solvent mixture a magnesium halide compound selected from the group consisting of magnesium halide complexes of magnesium halide with water or alcohol and derivatives of magnesium halide wherein a halogen atom is replaced by a hydrocarboxyl group or a halohydrocarboxyl group; the solvent mixture consisting of an organic epoxy compound, selected from the group consisting of oxides of aliphatic olefins and diolefins, oxides of halogenated aliphatic olefins and diolefins and glycidyl ethers, all having 2 to 8 carbon atoms, and an organic phosphorus compound selected from the group consisting of alkyl phosphates, aryl phosphates, aralkyl phosphates, alkyl phosphites, aryl phosphites and aralkyl phosphites where alkyl has one to four carbon atoms and aryl has six to ten carbon atoms to form a homogeneous solution;
b) mixing the homogeneous solution with a liquid titanium compound having the formula TiXn(OR)4-n wherein X is halogen, R is an alkyl group being identical or different and n is an integer of 0 to 4;
c) adding at least one auxiliary precipitant selected from the group consisting of carboxylic acid anhydrides, carboxylic acids, ethers and ketones to form a precipitate;
d) adding a polycarboxylic acid ester when a precipitate appears;
e) separating the precipitate from the mixture and treating the separated precipitate with titanium compound, TiXn(OR)4-n wherein X is halogen, R is a hydrocarbon group and may be identical or different, and n is and integer of from 0 to 4, or a mixture thereof, in an inert diluent; and
f) washing the treated precipitate with an organic inert diluent as disclosed in U.S. Pat. Nos. 4,861,847 and 4,784,983, the disclosures of which are hereby incorporated.
The preferred auxiliary precipitant useful in the formation of the catalyst is an organic carboxylic acid anhydride, more preferably phthalic acid anhydride, added in the amount of 0.03 to 1.0 moles per mole of magnesium halide. The preferred polycarboxylic ester is selected from the group consisting of the esters of aromatic polycarboxylic acids, preferably dibutyl phthalate and di-isobutyl phthalate. The preferred inert diluent for washing the precipitate is 1,2-dichloroethane. Suitable titanium tetrahalides include TiCl4, TiBr4 and Til4, with TiCl4 being preferred.
As the present invention relates particularly to external electron donors, the term xe2x80x9celectron donorxe2x80x9d as used herein, refers to the external donor. The external electron donor acts as a stereoregulator to control the amount of atactic form of polymer produced. It may also increase the production of isotactic polymers. Organic silicon compounds are known in the art for use as electron donors. Examples of electron donors that are organic silicon compounds are disclosed in U.S. Pat. Nos. 4,218,339; 4,395,360; 4,328,122; 4,473,660 and 4,927,797. As mentioned, particular catalyst may produce better results when paired with a particular group of electron donors. Examples of this pairing of catalyst and electron donors are disclosed in U.S. Pat. Nos. 4,562,173; 4,547,552; and 4,927,797.
The electron donor is of the general formula SiRm(ORxe2x80x2)4-m where R is selected from the group consisting of an alkyl group, a cycloalkyl group, an aryl group and a vinyl group; Rxe2x80x2 is an alkyl group; and m is 0-3, wherein when R is an alkyl group, R may be identical with Rxe2x80x2; when m is 0, 1 or 2, the Rxe2x80x2 groups may be identical or different; and when m is 1, 2 or 3, the R groups may be identical or different. Preferably, the electron donor is selected from a organosilicon compound of the following formula: 
wherein R1 is an alkyl or cycloalkyl group containing primary, secondary or tertiary carbon atom attached to the silicon; R2 and R3 are alkyl or aryl groups; and R4 is an alkyl or cycloalkyl group with a secondary or tertiary carbon atom attached to the silicon atom, R1 and R4 being the same or different and R2 and R3 being the same or different. In a preferred embodiment, R1 is a secondary alkyl group of 3-5 carbon atoms having mirror image symmetry; R2 and R3 are methyl, ethyl, propyl, or butyl groups and not necessarily the same; and R4 is secondary alkyl or cycloalkyl group of 3-6 carbon atoms having mirror image symmetry. More preferably, R1 and R4 are the same and are a cycloalkyl group, specifically a group of 5 carbon atoms, i.e. cyclopentyl. Specific electron donors are cyclohexylmethyldimethoxysilane (CMDS), disopropyldimethoxysilane (DIDS), di-t-butyldimethoxysilane (DTDS), dicyclopentyldimethoxysilane (CPDS) and cyclohexylisopropyldimethoxysilane (CIDS), most preferred being dicyclopentyl dimethoxysilane (CPDS). The combination of the electron donor described above and a conventional supported Ziegler-Natta catalyst yields wholly unexpected results that surpass previously known catalyst systems. The electron donors as described by the present invention may be limited by the stability of the compound and the ease of handling including storage, transportation and use in the plant.
The catalyst system is capable of achieving high efficiencies over a wide range of Si/Ti ratios (4-200) while exhibiting a control of the formation of atactic polymer, or xylene solubles, at relatively low levels, resulting in a high isotactic index. When another catalyst is paired with these above described electron donors, the polymer product exhibits a sharp increase in the xylene solubles. The present catalyst system using a particular supported Ziegler-Natta catalyst component with CMDS, DIDS, DTDS, CPDS or CIDS, however, exhibits low xylene solubles from about 0.6 to about 3.0% as the Si/Ti ratio decreases from 200 to 4 and below.
The catalyst system with CPDS also provides higher crystallinity in the polymer product. The per cent of meso pentads (mmmm) of the heptane insoluble fraction of the polymer product is very high, being equal to or greater than 97%.
The present invention also provides a process for the polymerization of propylene using the catalyst and the electron donors described by the above formula comprising:
a) selecting a conventional supported Ziegler-Natta catalyst component;
b) contacting the catalyst with an organoaluminum compound;
c) contacting the catalyst with an electron donor as described above;
d) introducing the catalyst into a polymerization reaction zone containing the organoaluminum compound, the electron donor and the monomer; and
e) extracting polymer product from the reactor.
Although the catalyst system may be used in almost any commercially known polymerization process, the preferred process of the present invention includes a pre-polymerization of the catalyst by contacting a small amount of monomer with the catalyst after the catalyst has been contacted with the electron donor. One pre-polymerization process is described in U.S. Pat. Nos. 4,767,735, and 4,927,797, incorporated by reference above. As provided in those disclosures, a carrier stream for the catalyst is provided, the catalyst is contacted with the co-catalyst or organoaluminum compound, the catalyst is contacted with the electron donor, the catalyst stream is contacted with a relatively small amount of the total amount of monomer to be polymerized, the catalyst stream passes through a tubular reactor, and the pre-polymerized catalyst and catalyst stream are introduced into the polymerization reaction zone. The electron donor may be contacted with the catalyst simultaneously with the co-catalyst. A polymer product may then be withdrawn from the reactor. In using the described catalyst with the electron donors described by the formula above, the catalyst may have an efficiency of above 30 kg/g-catxc2x7h while the Si/Ti mole ratio in the reaction is within the range 1-10. The polymer product may also be characterized by xylene solubles within the range 2-10 wt %.
The following Examples and Comparative Examples illustrate the present invention and its various advantages in more detail. The results are summarized in Tables 1-2. The catalyst used in the Examples was believed to be prepared with the materials and processes as disclosed in U.S. Pat. Nos. 4,861,847 and 4,784,983 and is sold by Catalyst Resources, Inc. under the tradename of xe2x80x9cLynx 1000xe2x80x9d. the catalyst used in the Comparative Examples was believed to be prepared with the materials and processes as disclosed in U.S. Pat. Nos. 4,927,797; 4,816,433 and 4,839,321 and is sold by Toho Titanium, Inc. under the tradename xe2x80x9cTHC-32Axe2x80x9d.